During the past year, we investigated OCT (optical coherence tomography) imaging as functional measurements of retinal activity. OCT has been widely used as a non-invasive tool to provide optical sections of biological tissues. Because the optics of the eye allows for high resolution of retinal structure, OCT is often used in the diagnosis and monitoring of retinal disease progression in the clinic and the laboratory. As OCT technique uses infrared light in generating image, it also provides a useful tool to study light-induced changes in the retinal structure and thickness. For normal C57BL/6J mice, the OCT image of eyes adapted to room light exhibited significant increases (6.1+0.8 um, n=13) in total retina thickness compared with the same eyes after overnight dark adaptation. These light-adapted retinal thickness changes occurred mainly in the outer retina, with the development of a hypo-reflective band between the RPE and photoreceptor-tip layers. Histological analysis revealed a light-evoked elongation between the outer limiting membrane and Bruchs membrane from 45.8+1.7 um in the dark (n=5) to 52.1+3.7 um (n=5) in the light. Light-adapted retinas showed an increase of actin staining in RPE apical microvilli at the same location as the hypo-reflective band observed in OCT images. Elongation of the outer retina could be detected even with brief light exposures, increasing 2.1+0.3 um after 15min (n=9), and 4.1+1.0 um after 2hr (n=6). Conversely, dark-adaptation caused outer retinal shortening of 1.4+0.4 um (n=7) and 3.0+0.5 um (n=8) after 15min and 2hr, respectively. In conclusion, Light-adaption induces an increase in the thickness of the outer retina and the appearance of a hypo-reflective band in the OCT image. This is consistent with previous reports of light-induced fluid accumulation in the sub-retinal space.